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I can't believe after all these years and all the gear I have built I am about to build another chip. I'll say it again because even I don't believe it - I'm building another chip amp. Why?

I sold a two stage tube preamp, with tube rectification, to an audio club member who has already bought a lot of my stuff. When the preamp drives his J loop amp (only a prototype ever built) the speaker protection activates when the heating cuts in. The J Loop amp (not sure if that is the correct spelling) is a chip amp with other "enhanced" circuitry. The combination of the tube preamp and J Loop amp is extremely good.

My friend Dave who has $5K 845 monoblock tube amps heard the combo (at his place) and now wants to sell the big "tubers" and build a chip amp. I want to build a true clone to compare with his kit bought chip amp. But I have to pullout something big to beat the 'net bought PCB based kit. Thus the better built mouse trap.

Back panel, chip module (not shown) and massive copper plate heat sink are finished. The 100% pure copper plate weighs 3.6kg (8lb and 12mm thick) and the chips will be mounted to it for resonance and cooling reasons. Like my original Synergy amp http://www.diyaudioprojects.com/Chip/Synergy-LM3875-Gainclone/ this amp will focus on short signal paths and cct. resonance control. Two 24V toroids will form part of the PS and will cause this 1U enclosed chip amp to weigh-in like a heavy weight.

The name of the this amp: SynergyII\Cu. The first Synergy is still doing time in it's original owner's home providing many hours of music pleasure. Let's hope this new build lives up to the name and fame.

Attachment:

SynergyII_Cu.jpg

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The amp had a hum and I just could not remove. I tried an external PS which worked. The Hum and the buzz went. I realised the external PS had a slightly lower PS by only five volts. Could this be the reason my amp hummed and buzzed???

Attachment:

Testing.jpg

Experimenting with different resistors, starting at 100ohms and going down to 0.22ohms that I could remove the hum and buzz by slightly lowering the B+\B-. In the end the best value was 6.8ohms. The PS sags slightly on heavy passages. I could stop this sag with extra 10,000uf filter caps but this gets away from the "Gain Clone" formula I wanted to follower.

The amp is up and working with 6.8ohm 5W resistors in the +\- power feeds. Note the 5W Rs on the rectifier board.

Attachment:

UnderTheBonnet.jpg

This has removed all the hum and buzz with only minimal power sagging at the chips. As you can see it is a compact build with the massive copper plate (3.6kg), minimal chip amp and dual 24V low profile toroid(s). The power cord has a ferrite choked and there is a power filter (top right-hand corner) installed. The diodes are Super Fast with soft recovery (3A) and are snubbed (0.01uf). Only 4.7uf caps provide a pre-filter at the PS and 1500uf caps (Panasonic and snubbed - 0.1uf) provide smoothing and storage right at the chips. All the signal path is silver soldered and silver wire (0.4mm) and silver plated copper wire is used in the signal path. No shieled wire or PCB are used! The earth to the input RCAs and for the speaker return is five-nines solid silver wire back to the star earth. The copper plated is suspended on two very small brass washers, above the bottom plate of the enclosure. This 1mm of separation is to ensure only the resonance from the copper plate is heard.

The Sound of Copper: I played a 24B\96K classical baritone piece to start with and warm-up the amp. I was not impressed. "Please give me the Silver Dragon(300BSE)". Then I played Claire Martin, Jenny Schliemann and Liz Springer (Australian local singer). WOW does this amp rock. All the things I hated in the classical piece I loved in these upbeat jazz and rock pieces. So how does copper sound with rock\pop\upbeat jazz: Blo**o#dy great!! It's bright, thump'n and pump'n. It's in your face, loud, gutsy and driv'n. I just need the wife to go out for, oh, I don't know, about 10 more hours so I can work through some raunchy pop and rock. The Dragon still holds court but this 9.6kg (21.12lb) bad boy needs some attention.

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Getting the hum and buzz out of the amp was one thing but as I have stated it didn't sound that good with classical music. What I was hearing in the amp was some metal-sounding resonance in the mid to upper-mid range. I can't remember my other chip amp producing this sound. It must be some sort of speaker\amp interaction. I wondered if changing the Q of the mid-range and tweeter network would change the sound and remove this resonance. Sure enough placing a 15ohm resistor in series with the mid\tweeter drivers killed the resonance and metal sound instantly.

The mid\treble was too forward but with the resistor in-place the sound was a little too bassy. So I experiment with the value of resistor to find 8.2ohms perfect. The whole balance of the sound was now spot-on with no resonances. I went back to some classical and even Opera music and now thoroughly enjoying the amp.

This amp has some stability issues. I tried some different Zobels and the amp got worse. Finally I have this amp on a tight leash. I'd coiled (3 turns) of insulated pure silver wire to connect amp output and binding post. By screwing a 4mm X 10mm mild steel screw into this I have created a small choke. This is helping to controlling instability in the amp. The amp now sounds bloody great. Before this sibilance had a slight whistle sound and the upper-mid sounded metallic.

*steals the copper plate and holds it like Gollum would 'the one ring' and calls it precioussssss*

SS amps, discrete or chip, traditionally don't like capacitive loads. Nor do they like inductive, but the heavy NFB loop tends to take care of that problem.

A good quality inductor on the output will help and is a near univeral fix. Using the screw kinds "screws" the quality factor, as it will disappear at HF. My suggestion is to steal an inductor with ferrite rod from a computer SMPS.

*steals the copper plate and holds it like Gollum would 'the one ring' and calls it precioussssss*

SS amps, discrete or chip, traditionally don't like capacitive loads. Nor do they like inductive, but the heavy NFB loop tends to take care of that problem.

A good quality inductor on the output will help and is a near univeral fix. Using the screw kinds "screws" the quality factor, as it will disappear at HF. My suggestion is to steal an inductor with ferrite rod from a computer SMPS.

Cheers!

I hear ya man. But the screw is so shinny and nice. Ferite null. I want shinny sound please.

Because this amp has so much gain and now I have no tubes in the signal path, a simple tube preamp may be in order. I liked The Black of the 4S tube amp series. This was the third I made and the first one Matt designed. I made about six of the 4S preamps but sold them all. I wish I had kept just one. Oh well get the soldering iron!

For all it’s difficulty of construction and issues resolving the oscillation problems the amp is an absolute joy to listen to. It allows a steady musical flow to reach and emit from the speakers and appears not to get in the way of the music being offered to it. I do not feel, however that it passes on anywhere near the detail of individual instruments like of any of my tube amps. SyngeryII\Cu “plays” loud and clear the violin, pure and clean but I don’t hear the resin on the strings or the air resonating inside the body of the violin. This can be said for all instruments.

All instruments sound beautiful but I don’t get that I’m hearing the air around them or inside them for that matter. Instead of that transparent ethereal hyper-detail I might get standing in front of an oboe, guitar or piano I get the sound without the detail. Better engineers will go to great lengths to record the air around an instrument and not just the sound the instrument makes. That type of recording does not appear to come through here. Being a DC amp bass is extend and controlled.

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